Method of Fabricating an RF Substrate with Selected Electrical Properties

ABSTRACT

Method for fabricating a textured dielectric substrate ( 400 ) for an RF circuit. The method can include the step ( 104 ) of selecting a plurality of dielectric substrate materials, each having a distinct combination or set of electrical properties that is different from the combination of electrical properties of every other one of dielectric substrate materials. Selecting a textured substrate patter ( 106 ) which is comprised of at least two types of distinct areas respectively having the distinct sets of electrical properties, with each distinct area dimensioned much smaller than a wavelength at a frequency of interest. Cutting the dielectric substrate materials ( 202, 204 ) into a size and shape consistent with the distinct areas of the selected pattern so as to form a plurality of dielectric pieces ( 206, 208 ). Arranging the dielectric pieces on a base plate ( 302 ) in accordance with the selected pattern to form the textured dielectric substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

The application is a divisional of co-pending U.S. patent applicationSer. No. 10/694,468 filed Oct. 27, 2003.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. F005521 between the Defense Advanced Research ProjectsAgency, the United States Naval Research Laboratory and HarrisCorporation.

BACKGROUND OF THE INVENTION

1. Statement of the Technical Field

The invention concerns dielectric substrates for RF circuits, and moreparticularly dielectric substrates with selectively tailored electricalproperties.

2. Description of the Related Art

RF circuits, including antennas, are commonly implemented on dielectricsubstrates. Materials commonly used for this purpose includecommercially available low and high temperatures cofired ceramics (LTCC,HTCC). For example, low temperature 951 cofire Green Tape™ from Dupont®is Au and Ag compatible, has a acceptable mechanical properties withregard to thermal coefficient of expansion (TCE), and relative strength.It is available in thicknesses ranging from 114 μm to 254 μm and isdesigned for use as an insulating layer in hybrid circuits, multichipmodules, single chip packages, and ceramic printed wire boards,including RF circuit boards. Similar products are available from othermanufacturers.

LTCC substrate systems commonly combine many thin layers of ceramic andconductors. The individual layers are typically formed from aceramic/glass frit that can be held together with a binder and formedinto a sheet. The sheet is usually delivered in a roll in an unfired or“green” state. Hence, the common reference to such material as “greentape”. Conductors can be screened onto the layers of tape to form RFcircuit elements antenna elements and transmission lines. Two or morelayers of the same type of tape is then fired in an oven. The firingprocess shrinks all of the dimensions of the raw part. Accordingly, itis highly important that the material layers all shrink in a precise,well defined way that will provide consistent results from one module tothe next.

Other materials commonly used as RF substrates include Teflon® PTFE(PolyTetraFluoroEthylene) composites of glass fiber, woven glass andceramics. Such products are commercially available from a variety ofmanufacturers. For example, Rogers Corporation of Chandler, Ariz. offerssuch products under the trade name RT/duroid including product numbers5880, 6002, and 6010LM. Unlike LTCC materials, these types of substancesdo not generally require a firing step before they can be used. Instead,they are typically provided in the form of rigid board material with aconductive metal ground plane formed on one side.

Due to the maturing of the antenna design process, the continuedimprovement of new antennas is most limited by the choice of substratematerials. However, development of new materials as proved difficult fora variety of reasons. One reason concerns certain incompatibilities ofthe physical properties associated with diverse materials that wouldotherwise be desirable to combine in a single dielectric composition.Often, the thermal coefficient of expansion (TCE), chemical propertiesof the materials, or sintering properties of the material may beinconsistent with one another. For example, different types of unfiredceramics such as Green Tape™ will not fire well together because ofdifferent chemical and physical properties of the various differenttypes of materials.

Still, new materials are needed for a variety of reasons. One suchreason relates to the limited variety of specific electrical propertiesthat are offered in commercially available dielectric substratematerials. Designers wishing to implement antennas or other RF circuitdesigns often find themselves constrained by the limitations ofmaterial. For example, it may be desirable in a particular instance toimplement an antenna array on a portion of a substrate having aparticular value of permittivity, permeability or loss tangent. Therequirements for these electrical properties can relate to form factor,electrical performance or other design issues. In any case, the limitedchoices of substrate materials that are presently available can requiredesign compromises that are preferably avoided.

SUMMARY OF THE INVENTION

The invention concerns a method for fabricating a textured dielectricsubstrate for an RF circuit. The method can include the step ofselecting a plurality of dielectric substrate materials. Each of thedielectric substrate materials can have a distinct combination or set ofelectrical properties that is different from the combination ofelectrical properties of every other one of dielectric substratematerials. For example, two dielectric substrate materials can differfrom each other with regard to the value of at least one electricalproperty such as permittivity, permeability or loss tangent. A patterncan be selected which is comprised of at least two types of distinctareas respectively having the distinct sets of electrical properties,with each distinct area preferably dimensioned much smaller than awavelength at a frequency of interest.

Subsequently, each of the dielectric substrate materials can be cut intoa size and shape consistent with the distinct areas of the selectedpattern so as to form a plurality of dielectric pieces. Finally, thedielectric pieces can be selectively arranged on a base plate inaccordance with the selected pattern to form the textured dielectricsubstrate. This can be accomplished using a computer controlled pick andplace machine.

A layer of adhesive is preferably disposed on the base plate prior tothe arranging step. The adhesive layer can subsequently be cured, forexample in a heating step. Further, at least one RF circuit element canbe screen printed on the textured dielectric substrate. The textureddielectric substrate thus formed can have at least one effectiveelectrical property at a frequency of interest that is different fromeach of the distinct sets of electrical properties associated with theindividual dielectric materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart that is useful for understanding a process forpracticing the present invention.

FIG. 2 is a top view illustrating two different types of dielectricboard materials undergoing a cutting process.

FIG. 3 is a top view of the two dielectric board materials of FIG. 2with the pieces being placed on a base to create a substrate with apredetermined mosaic pattern.

FIG 4 shows a top view of the substrate after all of the dielectricpieces have been arranged in a predetermined pattern.

FIG. 5 shows a cross-sectional view of the substrate in FIG. 4 takenalong line 6-6.

FIG. 6 shows the cross-sectional view of the substrate in FIG. 5 afterconductive traces have been formed on the surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Computer modeling has shown that dielectric substrates with tailoredelectrical properties can be achieved by creating a textured substrate.Textured substrates have a geometric pattern of distance areas formedwithin the substrate. The distinct areas can have electricalcharacteristics such as permittivity, permeability or loss tangent thatare different from one another. Because the dimensions of the patternand the distinct areas are electrically small, the net effect is asubstrate that appears to have effective dielectric properties that areactually a blend of those properties of each of the distinct areas.While computer modeling suggests that textured substrate materials holdmuch promise for future designs, the availability of suitable methodsfor fabrication of textured substrate materials have med the commercialviability of this approach. The present invention described in relationto FIGS. 1-6 concerns a process for fabricating a textured substratewith tailored electrical properties.

A flow chart illustrating the process is shown in FIG. 1. As illustratedtherein, the process can begin with step 102 by selecting one or moreeffective electrical properties that are desired for the substrate whichis to be formed. These can include, without limitation, effective valuesof permittivity, permeability and loss tangent. In step 104, a pluralityof different types of bulk substrate board materials are selected, eachhaving distinct electrical properties. As used herein, the term “bulk”refers to material sized in pieces that is relatively large, e.g. largerthan about a wavelength at a frequency of interest.

In step 106, the process continues with the selection of a substratepattern that includes at least two types of distinct areas, each havingdistinct sets of electrical properties corresponding to the selectedbulk dielectric board materials and dimensioned much smaller than awavelength at a frequency of interest. The ability to predict theoutcome of the effective electrical property of a textured substrate ata particular frequency band of interest has just recently becomepractical with advances in computer modeling and optimization. Ingeneral, the pattern selection can include choosing the size, shape andarrangement of two or more different types of dielectrics that are to becombined in a pattern to obtain a particular effective electricalproperty.

Those skilled in the art will recognize that to some extent, the orderof steps 104 and 106 could be reversed under certain circumstances andthe invention is not intended to be limited by the order shown inFIG. 1. In particular, the number of possible substrate texture patternsthat can be used far exceed the number of bulk dielectric boardsubstrates that are commercially available. Accordingly, it can be moreconvenient in many instances to first select the bulk dielectric boardsubstrates in step 104. Thereafter a suitable substrate pattern can bedetermined to give a desired blending effect in step 106.

As used herein electrical properties include any of several electricalcharacteristics commonly associated with dielectrics, includingpermittivity, permeability, and loss tangent. References to “effectiveelectrical properties” are used to distinguish the characteristicsexhibited by textured dielectric materials fabricated with the processesdescribed herein. Effective electrical properties of the texturedsubstrate are to be distinguished from the bulk electrical properties ofthe bulk dielectric board materials from which the textured substratesare formed.

Referring now to FIGS. 1 and 2, the the process can continue in step 108with the positioning each of the two or more different types of bulkdielectric board material sheets 202, 204 in a suitable holder orcassette. According to a preferred embodiment, the dielectric boardmaterial can be selected so as to have a thickness that is approximatelyequal or slightly larger than the desired dielectric thickness of thecompleted substrate. Commercially available materials that can be usedto form the dielectric board material sheets 202, 204 can include any ofa wide variety of different types of commercially available composites.For example, Teflon® PTFE (PolyTetraFluoroEthylene) composites of glassfiber, woven glass and ceramics can be used for this purpose. Suchproducts are commercially available from a variety of manufacturersincluding Rogers Corporation of Chandler, Ariz. Some specific types ofbulk dielectric boards available from Rogers Corporation includeRT/duroid product numbers 5880, 6002, and 6010LM. However, the inventionis not limited to any particular substrate board type or material.

FIG. 2 shows that the two sheets 202, 204 of dielectric board materialcan be automatically cut or diced into a number of small dielectricpieces 206 and 208. FIG. 3 is a top view of the sheets 202, 204 afterthe cutting process is complete as the pieces are being placed on base.The size and shape of the pieces are preferably selected to conform tothe texture or pattern determined in step 106. In general, however, thedielectric pieces should be electrically small relative to thewavelength of interest for the device in which the completed substrateis to be used. For example, typical length and width of the dielectricpieces can be in a range of between about 1/10 to 1/50 of a wavelength,although it should be understood that the invention is not so limited.Also it may be noted that as a matter of convenience, the dielectricpieces 206, 208 in FIG. 3 are shown as having a square shape. However,it should be understood that the process is not so limited and morecomplex and intricate shapes are also possible. For example, and withoutlimitation, the shapes may constitute triangles, rectangles, sawtoothpatterns, interdigitated patterns, interlocking crosses, and so on.

The automated cutting equipment used to form the dielectric pieces 206,208 can be provided with the sheets 202, 204 as input stock. Thus,dielectric board material sheets having a size of between about 1 inchto 6 inches on each side can be convenient, but the invention is not solimited. The sheets 202, 204 can be cut automatically with a dicing sawsuch as model no. 984-6 which is available from Kulicke & Soffa ofWillow Grove, Pa. Precision automated saws of the kind described hereinare more commonly used for singulating silicon semiconductor devices.However, when provided with a cutting blade appropriate for compositesof glass fiber, woven glass or ceramics, they are particularly wellsuited for the present process.

It is desirable to cut the sheets 202, 204 in a very precise manner soas to minimize gaps between the dielectric pieces 206, 208 when they areassembled in a pattern to form the completed substrate. Depending on thethickness of the substrate, gaps of more than a few mills can causevariations in the effective electrical characteristics of the completedsubstrate. In general, the precision of the cutting process should beselected so that gaps between dielectric pieces are not more than about10% of the overall thickness of the completed substrate. After cutting,the set of dielectric pieces 206, 208 can remain in the cassette orholder, or can be attached to a film adhesive to create a carrier mediumto temporarily secure the dielectric pieces in a ordered manner. In anycase, the set of dielectric pieces 206, 208 is preferably secured in amanner suitable for allowing the pieces to be efficiently accessed by apick and place machine for assembly in the next step of the process.

In step 111, an adhesive layer 304 can be applied to a base plate 302.Base plate 302 can be formed of any suitable base material, dependingupon the desired configuration of the substrate. For example the baseplate can be a conductor if it is desired to provide a ground plane onone surface of the substrate. Alternatively, the base plate can also beformed of a semiconductor or a dielectric material. The adhesive layer304 can be an electronic grade epoxy film adhesive. If the base plate302 is a conductor, it can be advantageous to select a conductive typeepoxy film adhesive. For example the adhesive film can be a silverfilled epoxy with 70% silver particles.

In step 112, the dielectric pieces 206, 208 are arranged on the baseplate 302 in accordance with the selected texture or pattern. In FIG. 3,the beginning of this process is shown with two dielectric pieces 206,208 placed on the adhesive layer 304 in an alternating pattern.Commercially available computer controlled pick and place machines canbe used for this step. Such equipment is available from a variety ofsources. The degree of precision required for picking an placing of thedielectric pieces 206, 208 will depend on the RF wavelength at thefrequency of interest. In general, placement accuracy should besufficient to ensure that the gap between the dielectric pieces is lessthan about 0.01 wavelength. Once the dielectric pieces have been placed,it is preferred to avoid sliding the individual pieces to preventadhesive from inadvertently being pushed between the dielectric pieces.This is especially important if adhesive layer 304 is a conductiveadhesive as such irregularities in the ground plane can produceinconsistent electrical properties in the textured substrate.

FIG. 4 is a top view showing the completed textured substrate 400 withall of the pieces 206, 208 placed on the adhesive layer 304 in analternating checkerboard pattern following completion of step 112. FIG.5 is a cross-sectional view of the textured substrate showing adhesivelayer 304 and base plate 302. After all of the fired dielectric pieces206, 208 are placed as shown in FIGS. 4 and 5, the epoxy adhesive layeris cured in step 114 to permanently secure the dielectric pieces inplace. Curing time and temperature for the adhesive layer will varydepending on the particular material that is selected.

After the adhesive layer 304 has been cured, the entire texturedsubstrate can be lapped or polished in step 116 to achieve the finalthickness of the dielectric substrate that is required for the design.This step is also useful for smoothing out any irregularities as betweenthe exposed outer surface of all the dielectric pieces to bring them toa consistent height. Polishing is preferably performed by machine usinga suitable polishing mediums for the amount of material to be removed.For example, 10 to 40 micron grit wet slurrys can be used effectivelyfor this purpose.

In step 118, one or more conductive elements 602, 604 can be screenprinted on the textured substrate 400. Conductive elements 602 604 areshown disposed on the textured substrate 400 in FIG. 6. The screenprinting on the array can be performed using a conventional electronicsgrade conductive epoxy or ink that cures in the range from 100° F. to125° F.

1. A method for fabricating a textured dielectric substrate for an RFcircuit comprising the steps of: selecting a plurality of dielectricsubstrate materials, each of said plurality of dielectric substratematerials having a distinct set of electrical properties different froma remainder of said dielectric substrate materials; selecting a patterncomprised of at least two types of distinct areas having said distinctsets of electrical properties and each distinct area dimensioned muchsmaller than a wavelength at a frequency of interest; cutting each ofsaid plurality of dielectric substrate materials into a size and shapeconsistent with said distinct areas to form dielectric pieces;selectively arranging said dielectric pieces on a base plate inaccordance with said pattern to form said textured dielectric substratehaving at least one effective electrical property at a frequency ofinterest that is different from each of said distinct sets of electricalproperties.
 2. The method according to claim 1 wherein said arrangingstep further comprises forming a single layer of said dielectric pieceson base plate in accordance with said pattern.
 3. The method accordingto claim 2 further comprising the step of screen printing at least oneRF circuit element on said textured ceramic dielectric substrate.
 4. Themethod according to claim 1 further comprising the step of disposing alayer of adhesive on said base plate prior to said arranging step. 5.The method according to claim 4 further comprising the step of curingsaid adhesive layer in a heating step.
 6. The method according to claim4 further comprising the step of screen printing at least one RF circuitelement on said textured dielectric substrate.
 7. The method accordingto claim 1 further comprising arranging said dielectric pieces using acomputer controlled pick and place machine.
 8. The method according toclaim 1 further comprising the step of screen printing at least one RFcircuit element on said textured dielectric substrate.
 9. The methodaccording to claim 1 further comprising the step of polishing a surfaceof said textured ceramic dielectric substrate.
 10. The method accordingto claim 8 further comprising the step of screen printing at least oneRF circuit element on said textured ceramic dielectric substrate.